1. Field of the Invention
The present invention relates to a method of forming a crystal semiconductor film. More particularly it relates to a method of forming a crystal semiconductor film, comprising forming a plurality of monocrystals on a deposition surface under control of their location and also under control of the position of grain boundaries and the size of the monocrystals, and then flattening the monocrystals formed.
The present invention can be applied in, for example, crystal semiconductor films utilized for electronic devices such as semiconductor integrated circuits and magnetic circuits, optical devices, magnetic devices, piezoelectric devices or surface acoustic devices and so forth.
2. Related Background Art
In the field of an SOI (silicon on insulation) technique by which a plurality of monocrystals are made to grow on an insulating substrate, a method, for example, is proposed which is based on a selective nucleation depending on the difference in nucleation density between surface materials (T. Yonehara et al. (1987), Extended Abstracts of the 19th SSDM.191). This crystal formation method is described with reference to FIGS. 2A to 2C. First, as shown in FIG. 2A, on a substrate material 201 having a surface 203 with a smaller nucleation density, a region 207 having a surface with a larger nucleation density than the surface 203 is disposed in appropriate diameter and intervals. If a predetermined crystal formation processing is applied to this substrate, a nucleus 209 of a deposit is produced only on the surface of the region 207 and no nucleation takes place on the surface 203 (FIG. 2B). Then, the surface of the region 207 is called a nucleation surface (S.sub.NDL), and the surface 203, a non-nucleation surface (S.sub.NDS). If the nucleus 209 produced on the nucleation surface 207 is further made to grow, it grows to a crystal grain 210 (FIG. 2C), and further grows over the non-nucleation surface 203 beyond the region of the nucleation surface 207. In a short time it comes into contact with a crystal grain 210' having grown from an adjacent nucleation surface 207', and a grain boundary 211 is thus formed. In this conventional crystal formation method, reports have been made on an example in which amorphous Si.sub.3 N.sub.4 is used in the nucleation surface 207, SiO.sub.2 is used in the non-nucleation surface 203, and a plural number of Si monocrystals are formed by CVD (see the above paper); and an example in which SiO.sub.2 is used in the non-nucleation surface 203, Si ions are implanted into the non-nucleation surface 203 by the use of focused ion beams so that a region serving as the nucleation surface 207 is formed, and a plural number of Si monocrystals are formed by CVD (The 35th Applied Physics Related Joint Lecture Meeting, 1988, 28p-M-9).
When, however, plural crystal grains are formed in a lattice dot form by the above crystal formation method in which the monocrystals are formed under control of their location, the following problems may often occur, and hence some difficulties may be caused in the fabrication of semiconductor integrated circuits or other devices.
That is to say, in the above crystal formation method, it is difficult to obtain flat crystals under the present state in the art and a greater part of the resulting crystals is in a mass form. However, in the case when an element is formed, it is very effective to flatten the top of the crystals, from the viewpoints of improving performance of the element, making uniform the characteristics and improving the yield. In addition, if one element is formed on each of these monocrystals and each element is formed separately from each other through an insulating material, a region usually required for the separation of elements can be remarkably reduced and the devices can be more highly integrated. In recent years, it has been found that the performance of an element can be improved when a semiconductor layer on an insulating material is made thinner (International Workshop on Future Electron Devices, 1988). Nonetheless, no satisfactory technique has been established for flattening the crystals.
As methods for obtaining a thin layer with a flat surface, three methods have been hitherto proposed. The first is a method in which the upper part of, for example, an Si layer is oxidized and then removed using an etchant of an acid type (i.e., an oxide layer removing method), the second is a method in which a layer is made thin by reactive ion etching, and the third is a selective mechanochemical abrasive method in which a special chemical polishing solution is mixed and the remarkable difference in abrasion rates between Si and SiO.sub.2 is utilized (i.e., mechanochemical polishing method) (see T. Hamaguchi and N. Endo, Japanese Journal of Applied Physics, Vol. 56, No. 11, p.1480; T. Hamaguchi, N. Endo, M. Kimura and A. Ishitani, Japanese Journal of Applied Physics, Vol. 23, No. 10, 1984, PD.LO-815; T. Hamaguchi, N. Endo, M. Kimura and M. Nakamae, Proceeding of International Electron Device Meeting, p.688, 1985, Washington D.C., U.S.A.).
The oxide layer removing method must use a process such as high-pressure oxidation so that the rate of oxidation can be accelerated, resulting in a very expensive process. In addition, when the surface of a starting material is irregular, the surface must be flattened by any method before it is oxidized.
Moreover, in the case when grain boundaries or crystals with different orientation are present in the Si layer, there are many problems in achieving uniform and flat oxidation, since the oxidation may be accelerated along the grain boundaries or the rate of oxidation may be anisotropic depending on the crystal orientation.
The second method, the reactive ion etching, requires regulation of etching time to carry out thickness control in order to obtain a thin layer in a desired thickness, and there are many problems in its controllability, reproducibility, uniformity and mass productivity for accurately controlling the crystals to have a film thickness of 1 .mu.m or less. Moreover, what is to be additionally questioned is that ions having energy are made directly incident on the surface of semiconductor crystals. This leaves the problem of damaging the surface layer.
In the last mechanochemical polishing method, when used in usual silicon wafers, polishing is carried out using a polishing solution and a polyurethane cloth. The polishing solution is obtained by suspending abrasive grains called colloidal silica, comprising SiO.sub.2 with a diameter of about 0.01 .mu.m, in a weakly alkaline solution. This is a polishing method in which the physical polishing action attributable to the friction between abrasive grains (SiO.sub.2) and a silicone wafer is combined with the chemical dissolving action of silicon in the weakly alkaline polishing solution because of an increase in temperature during the friction. The mechanochemical polishing is used in a final step when substrates such as silicon wafers are polished, and the surface of the substrate thus polished is a flat, strain-free mirror surface.
A selective polishing technique based on this mechanochemical polishing method employs a weakly alkaline solution as a processing solution, and utilizes the mechanism that the chemical reaction between the solution and a body to be polished differs depending on materials. This method is comprised of the chemical reaction of this solution and a mechanical removing step so that a substance to be formed is rubbed off with a polishing cloth. For example, when Si is etched using ethylenediamine-pyrocatechol, Si(OH).sub.6.sup.2- is formed on the Si surface as a result of redox reaction in the course of ionization of amine, and it produces a chelate with pyrocatechol, which is then dissolved in the solution. It is through the selective polishing that this Si(OH).sub.6.sup.2- is removed from the Si surface by the mechanical action attributable to the fiber of the polishing cloth. In the case when the body to be polished is comprised of SiO.sub.2 regions and Si regions, the effect of rubbing off using the polishing cloth may remarkably decrease if the Si regions surrounded by the SiO.sub.2 regions are abraded to the height of the SiO.sub.2 regions. Thus, the SiO.sub.2 regions are made to serve as a stopper and only the Si regions are flatly polished.
In the above ordinary mechanochemical polishing technique and selective mechanochemical polishing technique, a chemical reaction step is present and hence a remarkable difference in abrasion rate, depending on the crystal orientation of Si, can be observed in both instances.
For example, it is known that in the case of the ordinary mechanochemical polishing technique the (100) face can be processed at a rate which is greater by 10 to 20% than the (111) face, and in the case of the selective mechanochemical polishing technique the (100) face and (110) face can be polished at a rate which is 10 times as fast as the (111) face.
Such dependence of abrasion rates on the face orientation is not a problem in the case of a large-area monocrystalline substrate having perfectly uniform face orientation as in the case of a bulk Si substrate. However in the case of an Si thin layer formed on an amorphous insulating substrate, it is very rare that a layer with perfectly uniform face orientation. In many instances, formed is a thin film is formed, comprised of an aggregate of polycrystal grains with more or less non-uniformity in face orientation or an aggregate of relatively large single crystals in a mosaic form having grain sub-boundaries. In such instances in which the face orientation is not perfectly uniform or grain boundaries, grain sub-boundaries or twin crystal grain boundaries are present, it is very difficult to obtain flat surfaces by the mechanochemical polishing process having a chemical factor, because of its face orientation dependence. In addition, chemical etching is accompanied with acceleration reaction that may take place at a defective region, where the processing preferentially proceeds at the part in which grain boundaries and so forth are present, resulting in further deterioration of surface flatness.
It has recently been sought that a photoelectric transducer be formed on a transparent substrate to make it serve as an image input part of a facsimile, etc. or that an element be formed on a large, inexpensive glass substrate. In particular, if monocrystals are formed on such a substrate in a mutually separate form, it is possible to increase the performance of an element and expand the scope of its utilization because the same characteristics as those of an element on a bulk semiconductor can be exhibited. The above crystal formation method that forms monocrystals under control of their location is a crystal formation method which is very effective also in this respect. When a group of crystals obtained by such a method are flattened, the problems as pointed out in the above have often occurred even with use of the polishing process as described above. Hence, it has been difficult to form a thin layer with greater flatness, with better accuracy (with a film thickness of about 1 .mu.m or less) and also with less non-uniformity.